1. Field of the Invention
The present invention relates to a method and apparatus for performing modal mass analysis of an exhaust gas from a motor vehicle subjected to varying driving modes during a mass emissions test.
2. Description of the Prior Art
Many countries require the certification of motor vehicles, especially with respect to engine emissions. Many of the prescribed certification processes require a so-called CVS (constant-volume sampling) dilution system for performing a modal mass analysis. In a CVS-based modal mass analysis procedure, a sample is taken from a predetermined quantity of gas, composed of engine exhaust and ambient air. The ratio between the exhaust gas and air changes continually because the driving cycle involves different driving modes, such as acceleration, deceleration, etc. of the vehicle. Each driving mode results in different exhaust-gas/air ratios.
A method of modal mass analysis of an exhaust gas from a motor vehicle utilizing a CVS for calculation of flow values QE(t), during a mass emissions test, is disclosed in U.S. Pat. No. 4,727,746, to Mikasa, et al. Until now, the basic steps for performing a modal mass analysis of the exhaust gas has involved:
(a) connecting the vehicle under test to a dynamometer in a conventional manner in order to control vehicle load (i.e., subject the vehicle's engine while in gear to acceleration, deceleration, cruise speed conditions, etc.) and attaching some means to read vehicle speed/distance; PA1 (b) connecting a CVS system to the vehicle's exhaust to establish a constant flow rate; PA1 (c) further connecting a concentration analyzer to the vehicle exhaust to measure concentration levels of a specified ingredient gas CE(t) in real time; and PA1 (d) coupling an engine analyzer to the engine, the dynamometer, the CVS system and to the concentration analyzer--the engine analyzer controls the dynamometer to visually guide the driver through the transient test, and computes mass emission levels on the basis of the measured flow rate values QE(t) and concentration levels CE(t) from the CVS and the concentration analyzer, respectively.
Exhaust flow values are critical in calculating mass emission levels in a modal mass analysis. Equally important are the concentration levels of ingredient gases, such as CO, CO2, NOx, HC and the like, determined at specified intervals over the sampling period. The Mikasa et al. patent discloses the well-known relationship that the quantity (mass) of any particular ingredient gas in the exhaust gas is determined for each driving mode by the use of the following operational equation: EQU M(t)=.rho..times.C.sub.E (t).times.Q.sub.E (t), eq. (1)
where .rho. is the density of the ingredient gas, CE(t) is the concentration level of the ingredient gas at time (t), and QE(t) is the flow rate of the exhaust gas at some time (t).
Theoretically, the exhaust flow rate is a function of engine displacement and engine speed and expressed by: EQU FLOW-theo(QEi)=displacement.times.(1/1728).times.(RPM/60), eq. (2)
where the expression (1/1728) converts the cubic inch displacement into a cubic foot displacement, and the expression (RPM/60) converts revolutions per minute into revolutions per second, to yield consistent flow rate parameters. Unfortunately, the theoretical flow calculation differs drastically from the actual flow. There having been, until now, no recognizable linear or non-linear relationship between the theoretical and actual flows over varying driving conditions. Under some driving conditions, there may be as much as a 30-50%, or more, difference between theoretical and actual flow values.
Given recently required improvements in the performance of motor vehicles in view of governmental regulations of exhaust gas emissions, test requirements have become more stringent. As a result, all stations must now use an expensive CVS device or the functional equivalent thereof to conduct emissions testing. A CVS typically works by creating a flow that is higher than the exhaust flow on any car (typically 750 CFM). The exhaust flow passes through an orifice of a venturi, creating a differential pressure which is measured, over a sampling period, with two pressure transducers. On the basis of the measured differential pressure values and the relative dimensions of the CVS, such as the diameter of the orifice and the CVS throat diameter, an exhaust flow rate can be calculated, using text book formulas, which is within a few percentage points of actual flow.
However, given the critical dimensioning in design and the cost of component materials necessary for construction of a CVS, such devices are extremely expensive and non-cost-effective for use with decentralized type test sites, such as in states which allow vehicle owners to have their vehicle inspected at a local, authorized, privately-owned repair station, as opposed to a centralized government test facility.
It would be a great advancement in the art of mass emissions testing to be able to determine mass quantity of a gas without the use of a CVS.